Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Thousands		12 Months Ended
	Apr. 29, 2014	Dec. 31, 2016 USD ($) charge_off	Dec. 31, 2015 USD ($) charge_off	Dec. 31, 2014 USD ($) charge_off
Allowance for Loan Losses
Beginning balance		$ 112,507	$ 83,842	$ 68,081
Total provision		159,339	88,349	85,529
Charge-offs		(91,551)	(57,939)	(17,438)
Recoveries		10,382	5,820	1,155
Net charge-offs		(81,169)	(52,119)	(16,283)
Student loan sales		(6,034)	(7,565)	(53,485)
Ending Balance		184,643	112,507	83,842
Allowance:
Ending balance: individually evaluated for impairment		86,930	43,480	9,815
Ending balance: collectively evaluated for impairment		97,713	69,027	74,027
Loans:
Ending balance: individually evaluated for impairment		612,606	265,831	59,402
Ending balance: collectively evaluated for impairment		14,649,977	11,446,269	9,516,781
Period of delinquent loans sold to subsidiary (greater than)	90 days
FFELP Loans
Allowance for Loan Losses
Beginning balance		3,691	5,268	6,318
Total provision		(172)	1,005	1,946
Charge-offs		(1,348)	(2,582)	(2,996)
Recoveries		0	0	0
Net charge-offs		(1,348)	(2,582)	(2,996)
Student loan sales		0	0	0
Ending Balance		2,171	3,691	5,268
Allowance:
Ending balance: individually evaluated for impairment		0	0	0
Ending balance: collectively evaluated for impairment		2,171	3,691	5,268
Loans:
Ending balance: individually evaluated for impairment		0	0	0
Ending balance: collectively evaluated for impairment		$ 1,010,908	$ 1,115,663	$ 1,264,807
Net charge-offs as a percentage of average loans in repayment		0.17%	0.30%	0.31%
Allowance as a percentage of the ending total loan balance		0.21%	0.33%	0.42%
Allowance as a percentage of the ending loans in repayment		0.28%	0.45%	0.57%
Allowance coverage of net charge-offs | charge_off		1.61	1.43	1.76
Ending total loans, gross		$ 1,010,908	$ 1,115,663	$ 1,264,807
Average loans in repayment		793,203	857,359	972,390
Ending loans in repayment		786,332	813,815	926,891
Private Education Loans
Allowance for Loan Losses
Beginning balance		108,816	78,574	61,763
Total provision		159,511	87,344	83,583
Charge-offs		(90,203)	(55,357)	(14,442)
Recoveries		10,382	5,820	1,155
Net charge-offs		(79,821)	(49,537)	(13,287)
Student loan sales		(6,034)	(7,565)	(53,485)
Ending Balance		182,472	108,816	78,574
Allowance:
Ending balance: individually evaluated for impairment		86,930	43,480	9,815
Ending balance: collectively evaluated for impairment		95,542	65,336	68,759
Loans:
Ending balance: individually evaluated for impairment		612,606	265,831	59,402
Ending balance: collectively evaluated for impairment		$ 13,639,069	$ 10,330,606	$ 8,251,974
Net charge-offs as a percentage of average loans in repayment		0.96%	0.82%	0.30%
Allowance as a percentage of the ending total loan balance		1.28%	1.03%	0.95%
Allowance as a percentage of the ending loans in repayment		1.88%	1.57%	1.53%
Allowance coverage of net charge-offs | charge_off		2.29	2.20	5.91
Ending total loans, gross		$ 14,251,675	$ 10,596,437	$ 8,311,376
Average loans in repayment		8,283,036	6,031,741	4,495,709
Ending loans in repayment		$ 9,709,758	$ 6,927,266	$ 5,149,215